as3 cellrenderer checkbox selected

i'm trying to use ComboBox with a list of checkboxes. It's working fine, but i can't start the list with all checkboxes selected. i thought i could add items with the dataProvider "additem" method, and define selected:true, but it's not working. Is it possible to start this CheckBox list in a ComboBox with all checkbox selected?

Something like this:

package eines
{
import flash.display.MovieClip;
import fl.controls.ComboBox;
import fl.data.DataProvider;
import flash.text.TextFormat;
import eines.CustomCellRenderer;

public class CheckComboBox extends MovieClip
{

    public var myCB:ComboBox = new ComboBox();
    public var myDP:DataProvider = new DataProvider();

    public function CheckComboBox(dades:Array, missatge:String, amplada:Number, font_name:String = "Calibri", font_size:Number = 10) 
    {
        var myformat:TextFormat = new TextFormat();
        myformat.font = font_name;
        myformat.size = font_size;

        for(var i:int = 0; i<dades.length; i++)
        {
            myDP.addItem({label:dades[i], ***selected:true***, data:i});
        }
        myCB.dataProvider = myDP;
        myCB.prompt = missatge;
        myCB.width = amplada;
        myCB.dropdown.setStyle("cellRenderer", CustomCellRenderer);
        myCB.textField.setStyle("textFormat", myformat); 
        myCB.dropdown.setRendererStyle("textFormat", myformat);
        addChild(myCB);

    }
}
}